Conventionally, an atomic clock (or atomic oscillator) is known as a clock that provides very precise timing. Recently, research and development for miniaturizing the atomic clock has actively been made. The atomic clock may be considered an oscillator that uses an electronic transition frequency in the microwave region of the electromagnetic spectrum of atoms, such as alkali-metal atoms, as a frequency standard on the basis of the amount of transition energy of electrons in the atoms. In particular, in the absence of external influences, a very precise value of timing can be obtained from the transition energy of the electrons in the alkali-metal atoms. The frequency stability of the atomic clock of this type is at several orders of magnitude better than those of the existing crystal oscillators. For example, see Japanese Patent No. 4,801,044; “The MAC—A Miniature Atomic Clock” by R. Lutwak, P. Vlitas, M. Varghese, M. Mescher, D. K. Serkland and G. M. Peake, Joint Meeting of the IEEE International Frequency Control Symposium and the Precise Time and Time Interval (PTTI) Systems and Applications Meeting, Vancouver, BC, Canada, pp. 752-757 (2005); and “Investigations on Continuous and Pulsed Interrogation for a CPT Atomic Clock” by N. Castagna, R. Boudot, S. Guérandel, E. De Clercq, N. Dimarcq and A. Clairon, IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, Vol. 56, No. 2, February 2009.
There are some types of the atomic clock. For example, the conventional type utilizing a microwave resonator requires a large device size and high power consumption. However, a CPT (coherent population trapping) atomic clock is capable of providing a very high frequency stability which is at three orders of magnitude better than that of the existing crystal oscillator. Further, the CPT atomic clock is able to provide a small device size and low power consumption. A CPT atomic clock prototype was produced in 2007 and the product of chip-scale atomic clock (CSAC) of this type became available commercially in 2011 by Symmetricom.
Although the CPT atomic clock described above has a power consumption lower than that of the conventional atomic clock utilizing the microwave resonator, the power consumption of the above CPT atomic clock was approximately 115 mW. If the practical use of an atomic clock in battery-driven portable applications is taken into consideration, the power consumption value of the above CPT atomic clock is still too high. Accordingly, there has been a demand for an improved atomic clock or oscillator with a lower power consumption of less than 30 mW.